1. Field of the Invention
The field of invention relates to microporous carbons. More specifically, the field relates to the formation of microporous carbons and use in natural gas storage and transportation systems.
2. Description of the Related Art
Natural gas is the portable and preferred fuel of choice around the world. Natural gas burns more completely than other traditional fuels, including petroleum and coal; therefore, the combustion of natural gas is comparatively less harmful to the environment. Natural gas and similar products, including LNG, propane and other compressed-gas fuels, are much more efficient in engine and turbine combustion systems. Pipelines are the traditional and most cost-effective means of transporting natural gas from the producer to the consumer.
When producing electricity or natural gas for non-commercial users, a significant problem arises for natural gas transportation networks: diurnal demand. People, unlike manufacturing plants or facilities, tend not to be steady energy users throughout the day. People consume greater amounts of electricity during the day and into the early evening and much less at night and into the early morning. The higher rates of consumption form a “peak period of demand” and the lower rate of consumption creates a “non-peak period of demand”. This daily trend occurs throughout the year. During different seasons, however, the length of each period (longer or shorter periods of natural light requiring reduced or greater amounts of artificial light, respectively) and the amplitude of the period (for example, greater amounts demanded at higher and lower temperatures versus more moderate temperatures) can change the amount of demand during the diurnal period. The location of the demand also has an impact upon the diurnal demand. In cooler environments, overall daily electrical and natural gas demand is lower in the summer months and higher in winter months as consumers use heating equipment. In warmer environments, the daily demand trends are opposite as consumer use air conditioning units when it is hot.
Swinging electrical and natural gas consumption—not only in daily use but also in seasonal differences—results in variability across the natural gas transportation and production system. Natural gas production, however, is nearly constant. The supply-demand gap between natural gas production and total consumption results in a “gas demand lag”. The lag, without intervention, manifests itself as system pressure increases and decreases (“swings”) across the natural gas transportation grid.
Electrical generation facilities prefer constant, high-pressure natural gas as a feedstock. Pressure swings in natural gas feed can damage the electrical generation equipment, especially rotational equipment, including gas turbines, due to sudden inappropriate feed-to-fuel ratios that cause equipment slowdowns while under load.
A solution to mitigating the pressure swings in gas transportation networks is provided for in U.S. Pat. App. Pub. No. 2013/0283854 (published Oct. 31, 2013) (Wang, et al.), titled “Adsorbed Natural Gas Storage Facility”, which uses a microporous adsorbent to adsorb and desorb natural gas.
Microporous adsorbents for adsorbed natural gas (ANG) storage include activated carbons, metal-organic frameworks (MOFs), zeolites and other organic or inorganic porous solids. MOFs have been reported to have surface areas up to 4000 meters squared per gram (m2/g) and absolute methane adsorption capacities as high as 230 volume to volume (v/v) absolute methane adsorption at 290 K and 35 bar (sometimes referred to as the “storage amount” ratio or the “amount stored” ratio). There is some question, however, as to whether this high number of absolute methane adsorption is accurate. Several operational issues limit the practical use of MOFs in natural gas adsorption-desorption systems. Methane, once adsorbed into the framework, is strongly bound, so for desorption temperatures as high as 100° C. may be required to free the adsorbed methane. MOFs are known to have a reduced hydrothermal stability, so heating them to release methane repeatedly will eventually degrade the framework. MOFs also are intolerant to natural gas impurities such as hydrogen sulfide, black/carbon-silicone powder and mercaptans, which are common in natural gas.
Metal oxide adsorbents such as zeolites tend to adsorb less methane than activated carbon materials at similar conditions. MOs possess a smaller surface area—reportedly less than about 800 m2/g. Zeolites also have hydrophilic surfaces relative to activate carbon material that makes them adsorb water over other constituents in a natural gas stream.
Activated carbon materials have surface areas in a range up to about 3000 m2/g and are relatively thermally and chemically stable materials. Activated carbons are known in the industry to have an absolute methane adsorption capacity in a range of from about 130 to about 180 v/v methane adsorption at 290 K and 35 bar.
There are several limitations to using activated carbon materials in an ANG application. Activated carbon materials have generally a lower packing density than other materials due to the presence of meso- (2<d<50 nanometers (nm)) and macro-sized pores (>50 nm). Larger micropores are generated upon formation of the activated carbon material due to excessive carbon burn-off during the carbon activation process. The irregular morphology of carbon particles with high surface areas tends to cause the dense packing of particles, leaving voids between particles. The optimum pore diameter for ANG is from about 1.1 to about 1.2 nm. The meso- and macro-sized pores do not contribute to natural gas adsorption but do count as part of the material volume, resulting in lower packing density. Useful activated carbon materials have a bulk density is in a range of from about 0.20 to about 0.75 grams per cubic centimeter (g/cm3).
Another issue is slow mass transport through microporous materials. Activated carbon materials having microporous can exhibit slow kinetic adsorption-desorption behavior due to slow mass transport. Slow mass transport can be attributed to large micropore volumes with smaller-than-useful pore diameters for adsorbing methane and a lack of connectivity between surface pore aperture openings (also known as “dead end pores”). Pressure and temperature changes can help accelerate the mass transfer to and from the microporous material.
Another limitation is the number of potential materials useful to design the activated carbon materials. Activated carbon materials are produced by chemical combustion of non-porous carbon precursors in a controlled manner. Although this method provides an economic way of producing material in the macro sense of a controlled reaction, rational and systematic design of specific and regular carbon pore structures is not possible due to the highly variable combustion process on the micro level. Structure parameters including surface area, pore diameter and micropore volume are strongly related to one another and are difficult to control separately. As an example, a high degree of burn-off achieves a large carbon surface area, which is positive for increasing gas storage capacity. The high degree of burn-off, however, also results in the unavoidable enlargement of pore diameters, which decreases the adsorption strength and packing density of the adsorbents per unit volume.
It is desirable to develop a method for forming an activated carbon material, the activated carbon material, and a method of its use that maintains or improves upon the packing density, the mass transport and the adsorptive strength of activated carbon materials while maintaining or improving upon the surface area and absolute methane adsorption capacities of activated carbon materials. Ease of use and handling of the activated carbon material and simplicity of manufacturing are also desirable characteristics.